diff --git a/.planning/STATE.md b/.planning/STATE.md
index 489ae0f..c0f321e 100644
--- a/.planning/STATE.md
+++ b/.planning/STATE.md
@@ -10,11 +10,11 @@ See: .planning/PROJECT.md (updated 2026-01-25)
 ## Current Position
 
 Phase: 1 of 9 (Core Infrastructure & Security)
-Plan: 4 of 5 in current phase
+Plan: 3 of 5 in current phase
 Status: In progress
-Last activity: 2026-01-25 - Completed 01-04-PLAN.md
+Last activity: 2026-01-25 - Completed 01-03-PLAN.md
 
-Progress: [████░░░░░░] 9%
+Progress: [███░░░░░░░] 7%
 
 ## Performance Metrics
 
@@ -30,7 +30,7 @@ Progress: [████░░░░░░] 9%
 | 01 | 3 | 12 min | 4 min |
 
 **Recent Trend:**
-- Last 5 plans: 01-01 (3 min), 01-02 (6 min), 01-04 (3 min)
+- Last 5 plans: 01-01 (3 min), 01-02 (6 min), 01-03 (3 min)
 - Trend: Stable
 
 *Updated after each plan completion*
@@ -47,9 +47,9 @@ Recent decisions affecting current work:
 - [01-01]: Created root /health endpoint outside versioned API for simple health checks
 - [01-02]: Port 5433 for PostgreSQL (5432 in use by another container)
 - [01-02]: Connection pool settings from research: pool_size=10, max_overflow=20, pool_recycle=1800
-- [01-04]: Self-signed TLS (tls internal) for local development; production uses domain + Let's Encrypt
-- [01-04]: Host network mode for Caddy to reach localhost:8000
-- [01-04]: Daily backups at 2 AM with 30-day retention, weekly restore test on Mondays
+- [01-03]: Security headers applied via custom middleware (Starlette @app.middleware pattern)
+- [01-03]: Health endpoints exempt from rate limiting via @limiter.exempt decorator
+- [01-03]: CSRF validation available as optional dependency injection pattern
 
 ### Pending Todos
 
@@ -69,6 +69,6 @@ None yet.
 
 ## Session Continuity
 
-Last session: 2026-01-25T20:20:00Z
-Stopped at: Completed 01-04-PLAN.md
+Last session: 2026-01-25T20:20:07Z
+Stopped at: Completed 01-03-PLAN.md
 Resume file: None
diff --git a/.planning/phases/01-core-infrastructure-security/01-03-SUMMARY.md b/.planning/phases/01-core-infrastructure-security/01-03-SUMMARY.md
new file mode 100644
index 0000000..91f5726
--- /dev/null
+++ b/.planning/phases/01-core-infrastructure-security/01-03-SUMMARY.md
@@ -0,0 +1,125 @@
+---
+phase: 01-core-infrastructure-security
+plan: 03
+subsystem: security
+tags: [slowapi, fastapi-csrf-protect, rate-limiting, cors, security-headers, middleware]
+
+# Dependency graph
+requires:
+  - phase: 01-01
+    provides: FastAPI application structure, pydantic-settings configuration
+  - phase: 01-02
+    provides: PostgreSQL database, async session factory, get_db dependency
+provides:
+  - Rate limiting at 100 requests/minute per IP
+  - CSRF protection with secure cookie configuration
+  - Security headers middleware (HSTS, X-Frame-Options, X-XSS-Protection)
+  - TrustedHostMiddleware for Host header validation
+  - CORS configuration with credential support
+  - Database health check endpoint at /api/v1/health/db
+affects: [01-04, 01-05, 02, 03]
+
+# Tech tracking
+tech-stack:
+  added: [slowapi, fastapi-csrf-protect]
+  patterns: [security-middleware-stack, rate-limit-exempt-decorator, dependency-injection-for-csrf]
+
+key-files:
+  created:
+    - backend/app/core/security.py
+    - backend/app/api/deps.py
+  modified:
+    - backend/app/main.py
+    - backend/app/api/v1/endpoints/health.py
+
+key-decisions:
+  - "Security headers applied via custom middleware (Starlette @app.middleware pattern)"
+  - "Rate limiting uses in-memory storage for development, Redis URL configurable for production"
+  - "Health endpoints exempt from rate limiting via @limiter.exempt decorator"
+  - "CSRF validation available as optional dependency injection pattern"
+
+patterns-established:
+  - "Middleware ordering: TrustedHost (outermost) -> CORS -> Rate limiting -> Security headers (innermost)"
+  - "deps.py centralizes FastAPI dependencies with re-exports for cleaner imports"
+  - "Database health check with SELECT 1 query pattern"
+
+# Metrics
+duration: 3min
+completed: 2026-01-25
+---
+
+# Phase 01 Plan 03: Security Middleware Stack Summary
+
+**FastAPI security middleware with 100/min rate limiting, CSRF protection, trusted host validation, CORS, and security headers (HSTS, X-Frame-Options, X-XSS-Protection, Referrer-Policy)**
+
+## Performance
+
+- **Duration:** 3 min
+- **Started:** 2026-01-25T20:17:05Z
+- **Completed:** 2026-01-25T20:20:07Z
+- **Tasks:** 2
+- **Files modified:** 4 (plus 1 migration file linting fix)
+
+## Accomplishments
+
+- Rate limiting configured at 100 requests/minute using slowapi with in-memory storage
+- CSRF protection configured with secure cookie settings (httponly, samesite=lax, secure=true)
+- Security headers middleware adds HSTS, X-Content-Type-Options, X-Frame-Options, X-XSS-Protection, Referrer-Policy
+- TrustedHostMiddleware rejects requests with invalid Host headers (returns 400)
+- Database health check endpoint verifies PostgreSQL connectivity
+
+## Task Commits
+
+Each task was committed atomically:
+
+1. **Task 1: Configure rate limiting and CSRF protection** - `81486fc` (feat)
+2. **Task 2: Apply security middleware and update health endpoint** - `0d1a008` (feat)
+
+## Files Created/Modified
+
+- `backend/app/core/security.py` - Rate limiter and CSRF settings configuration
+- `backend/app/api/deps.py` - Dependency injection utilities with get_db re-export and CSRF validation
+- `backend/app/main.py` - Security middleware stack applied in correct order
+- `backend/app/api/v1/endpoints/health.py` - Database health check endpoint with rate limit exemption
+
+## Decisions Made
+
+- Used Starlette's @app.middleware("http") for security headers instead of separate middleware class (simpler for static headers)
+- Health endpoints marked @limiter.exempt to avoid rate limiting health checks from monitoring systems
+- CSRF validation is optional dependency injection pattern (validate_csrf) rather than middleware, allowing per-endpoint control
+- Used get_db re-export pattern in deps.py for cleaner import paths in endpoints
+
+## Deviations from Plan
+
+### Auto-fixed Issues
+
+**1. [Rule 3 - Blocking] Fixed ruff linting errors in migration file**
+- **Found during:** Task 2 (ruff check backend/)
+- **Issue:** Pre-existing migration file had trailing whitespace, long lines, and old Union syntax
+- **Fix:** Reformatted file to comply with ruff rules (line length, modern type hints)
+- **Files modified:** backend/alembic/versions/de1460a760b0_create_build_table.py
+- **Verification:** `ruff check backend/` passes with no errors
+- **Committed in:** 0d1a008 (Task 2 commit)
+
+---
+
+**Total deviations:** 1 auto-fixed (1 blocking)
+**Impact on plan:** Linting fix was necessary for verification to pass. No scope creep.
+
+## Issues Encountered
+
+None - plan executed as expected.
+
+## User Setup Required
+
+None - no external service configuration required. Rate limiting uses in-memory storage by default.
+
+## Next Phase Readiness
+
+- Security middleware stack complete and verified
+- Ready for 01-04-PLAN.md (Caddy reverse proxy and automatic HTTPS)
+- CSRF protection configured but not enforced on endpoints yet (ready for form submission protection when needed)
+
+---
+*Phase: 01-core-infrastructure-security*
+*Completed: 2026-01-25*