feat(sqlite): add SQLite writer for converting PostgreSQL schemas

- Implement SQLite DDL writer to convert PostgreSQL schemas to SQLite-compatible SQL statements.
- Include automatic schema flattening, type mapping, auto-increment detection, and function translation.
- Add templates for creating tables, indexes, unique constraints, check constraints, and foreign keys.
- Implement tests for writer functionality and data type mapping.

pkg/writers/sqlite/README.md

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+# SQLite Writer
+
+SQLite DDL (Data Definition Language) writer for RelSpec. Converts database schemas to SQLite-compatible SQL statements.
+
+## Features
+
+- **Automatic Schema Flattening** - SQLite doesn't support PostgreSQL-style schemas, so table names are automatically flattened (e.g., `public.users` → `public_users`)
+- **Type Mapping** - Converts PostgreSQL data types to SQLite type affinities (TEXT, INTEGER, REAL, NUMERIC, BLOB)
+- **Auto-Increment Detection** - Automatically converts SERIAL types and auto-increment columns to `INTEGER PRIMARY KEY AUTOINCREMENT`
+- **Function Translation** - Converts PostgreSQL functions to SQLite equivalents (e.g., `now()` → `CURRENT_TIMESTAMP`)
+- **Boolean Handling** - Maps boolean values to INTEGER (true=1, false=0)
+- **Constraint Generation** - Creates indexes, unique constraints, and documents foreign keys
+- **Identifier Quoting** - Properly quotes identifiers using double quotes
+
+## Usage
+
+### Convert PostgreSQL to SQLite
+
+```bash
+relspec convert --from pgsql --from-conn "postgres://user:pass@localhost/mydb" \
+                --to sqlite --to-path schema.sql
+```
+
+### Convert DBML to SQLite
+
+```bash
+relspec convert --from dbml --from-path schema.dbml \
+                --to sqlite --to-path schema.sql
+```
+
+### Multi-Schema Databases
+
+SQLite doesn't support schemas, so multi-schema databases are automatically flattened:
+
+```bash
+# Input has auth.users and public.posts
+# Output will have auth_users and public_posts
+relspec convert --from json --from-path multi_schema.json \
+                --to sqlite --to-path flattened.sql
+```
+
+## Type Mapping
+
+| PostgreSQL Type | SQLite Affinity | Examples |
+|----------------|-----------------|----------|
+| TEXT | TEXT | varchar, text, char, citext, uuid, timestamp, json |
+| INTEGER | INTEGER | int, integer, smallint, bigint, serial, boolean |
+| REAL | REAL | real, float, double precision |
+| NUMERIC | NUMERIC | numeric, decimal |
+| BLOB | BLOB | bytea, blob |
+
+## Auto-Increment Handling
+
+Columns are converted to `INTEGER PRIMARY KEY AUTOINCREMENT` when they meet these criteria:
+- Marked as primary key
+- Integer type
+- Have `AutoIncrement` flag set, OR
+- Type contains "serial", OR
+- Default value contains "nextval"
+
+**Example:**
+
+```sql
+-- Input (PostgreSQL)
+CREATE TABLE users (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(100)
+);
+
+-- Output (SQLite)
+CREATE TABLE "users" (
+    "id" INTEGER PRIMARY KEY AUTOINCREMENT,
+    "name" TEXT
+);
+```
+
+## Default Value Translation
+
+| PostgreSQL | SQLite | Notes |
+|-----------|--------|-------|
+| `now()`, `CURRENT_TIMESTAMP` | `CURRENT_TIMESTAMP` | Timestamp functions |
+| `CURRENT_DATE` | `CURRENT_DATE` | Date function |
+| `CURRENT_TIME` | `CURRENT_TIME` | Time function |
+| `true`, `false` | `1`, `0` | Boolean values |
+| `gen_random_uuid()` | *(removed)* | SQLite has no built-in UUID |
+| `nextval(...)` | *(removed)* | Handled by AUTOINCREMENT |
+
+## Foreign Keys
+
+Foreign keys are generated as commented-out ALTER TABLE statements for reference:
+
+```sql
+-- Foreign key: fk_posts_user_id
+-- ALTER TABLE "posts" ADD CONSTRAINT "posts_fk_posts_user_id"
+--   FOREIGN KEY ("user_id")
+--   REFERENCES "users" ("id");
+-- Note: Foreign keys should be defined in CREATE TABLE for better SQLite compatibility
+```
+
+For production use, define foreign keys directly in the CREATE TABLE statement or execute the ALTER TABLE commands after creating all tables.
+
+## Constraints
+
+- **Primary Keys**: Inline for auto-increment columns, separate constraint for composite keys
+- **Unique Constraints**: Converted to `CREATE UNIQUE INDEX` statements
+- **Check Constraints**: Generated as comments (should be added to CREATE TABLE manually)
+- **Indexes**: Generated without PostgreSQL-specific features (no GIN, GiST, operator classes)
+
+## Output Structure
+
+Generated SQL follows this order:
+
+1. Header comments
+2. `PRAGMA foreign_keys = ON;`
+3. CREATE TABLE statements (sorted by schema, then table)
+4. CREATE INDEX statements
+5. CREATE UNIQUE INDEX statements (for unique constraints)
+6. Check constraint comments
+7. Foreign key comments
+
+## Example
+
+**Input (multi-schema PostgreSQL):**
+
+```sql
+CREATE SCHEMA auth;
+CREATE TABLE auth.users (
+    id SERIAL PRIMARY KEY,
+    username VARCHAR(50) UNIQUE NOT NULL,
+    created_at TIMESTAMP DEFAULT now()
+);
+
+CREATE SCHEMA public;
+CREATE TABLE public.posts (
+    id SERIAL PRIMARY KEY,
+    user_id INTEGER REFERENCES auth.users(id),
+    title VARCHAR(200) NOT NULL,
+    published BOOLEAN DEFAULT false
+);
+```
+
+**Output (SQLite with flattened schemas):**
+
+```sql
+-- SQLite Database Schema
+-- Database: mydb
+-- Generated by RelSpec
+-- Note: Schema names have been flattened (e.g., public.users -> public_users)
+
+-- Enable foreign key constraints
+PRAGMA foreign_keys = ON;
+
+-- Schema: auth (flattened into table names)
+
+CREATE TABLE "auth_users" (
+    "id" INTEGER PRIMARY KEY AUTOINCREMENT,
+    "username" TEXT NOT NULL,
+    "created_at" TEXT DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE UNIQUE INDEX "auth_users_users_username_key" ON "auth_users" ("username");
+
+-- Schema: public (flattened into table names)
+
+CREATE TABLE "public_posts" (
+    "id" INTEGER PRIMARY KEY AUTOINCREMENT,
+    "user_id" INTEGER NOT NULL,
+    "title" TEXT NOT NULL,
+    "published" INTEGER DEFAULT 0
+);
+
+-- Foreign key: posts_user_id_fkey
+-- ALTER TABLE "public_posts" ADD CONSTRAINT "public_posts_posts_user_id_fkey"
+--   FOREIGN KEY ("user_id")
+--   REFERENCES "auth_users" ("id");
+-- Note: Foreign keys should be defined in CREATE TABLE for better SQLite compatibility
+```
+
+## Programmatic Usage
+
+```go
+import (
+    "git.warky.dev/wdevs/relspecgo/pkg/models"
+    "git.warky.dev/wdevs/relspecgo/pkg/writers"
+    "git.warky.dev/wdevs/relspecgo/pkg/writers/sqlite"
+)
+
+func main() {
+    // Create writer (automatically enables schema flattening)
+    writer := sqlite.NewWriter(&writers.WriterOptions{
+        OutputPath: "schema.sql",
+    })
+
+    // Write database schema
+    db := &models.Database{
+        Name: "mydb",
+        Schemas: []*models.Schema{
+            // ... your schema data
+        },
+    }
+
+    err := writer.WriteDatabase(db)
+    if err != nil {
+        panic(err)
+    }
+}
+```
+
+## Notes
+
+- Schema flattening is **always enabled** for SQLite output (cannot be disabled)
+- Constraint and index names are prefixed with the flattened table name to avoid collisions
+- Generated SQL is compatible with SQLite 3.x
+- Foreign key constraints require `PRAGMA foreign_keys = ON;` to be enforced
+- For complex schemas, review and test the generated SQL before use in production