feat(backend): Add pgvector extension migration

- Add Alembic migration to enable pgvector PostgreSQL extension
- Required for RAG knowledge base and embedding storage

Implements #19

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/alembic/versions/0003_enable_pgvector_extension.py

@@ -0,0 +1,66 @@
+"""Enable pgvector extension
+
+Revision ID: 0003
+Revises: 0002
+Create Date: 2025-12-30
+
+This migration enables the pgvector extension for PostgreSQL, which provides
+vector similarity search capabilities required for the RAG (Retrieval-Augmented
+Generation) knowledge base system.
+
+Vector Dimension Reference (per ADR-008 and SPIKE-006):
+---------------------------------------------------------
+The dimension size depends on the embedding model used:
+
+| Model                      | Dimensions | Use Case                    |
+|----------------------------|------------|------------------------------|
+| text-embedding-3-small     | 1536       | General docs, conversations  |
+| text-embedding-3-large     | 256-3072   | High accuracy (configurable) |
+| voyage-code-3              | 1024       | Code files (Python, JS, etc) |
+| voyage-3-large             | 1024       | High quality general purpose |
+| nomic-embed-text (Ollama)  | 768        | Local/fallback embedding     |
+
+Recommended defaults for Syndarix:
+- Documentation/conversations: 1536 (text-embedding-3-small)
+- Code files: 1024 (voyage-code-3)
+
+Prerequisites:
+--------------
+This migration requires PostgreSQL with the pgvector extension installed.
+The Docker Compose configuration uses `pgvector/pgvector:pg17` which includes
+the extension pre-installed.
+
+References:
+-----------
+- ADR-008: Knowledge Base and RAG Architecture
+- SPIKE-006: Knowledge Base with pgvector for RAG System
+- https://github.com/pgvector/pgvector
+"""
+
+from collections.abc import Sequence
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "0003"
+down_revision: str | None = "0002"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+
+def upgrade() -> None:
+    """Enable the pgvector extension.
+
+    The CREATE EXTENSION IF NOT EXISTS statement is idempotent - it will
+    succeed whether the extension already exists or not.
+    """
+    op.execute("CREATE EXTENSION IF NOT EXISTS vector")
+
+
+def downgrade() -> None:
+    """Drop the pgvector extension.
+
+    Note: This will fail if any tables with vector columns exist.
+    Future migrations that create vector columns should be downgraded first.
+    """
+    op.execute("DROP EXTENSION IF EXISTS vector")