feat: tiered API plans (Free/Bronze/Gold/Platinum)

- Free: $0, 30 req/min, market data only
- Bronze: $29/mo, 100 req/min, + companies & search
- Gold: $99/mo, 500 req/min, + filings & historical
- Platinum: $299/mo, 2000 req/min, all features + priority
- Plan-aware rate limiting per API key (or per IP for free)
- Usage tracking with daily aggregation
- GET /api/v1/plans — plan listing
- POST /api/v1/plans/register — instant free API key
- GET /api/v1/plans/usage — usage stats
- /pricing — dark-themed HTML pricing page
- X-RateLimit-* and X-Plan headers on every response
- Restricted endpoints return upgrade prompt
- Updated OpenAPI spec with security scheme
- 53 handlers, compiles clean

internal/db/schema.go

@@ -70,4 +70,38 @@ CREATE VIRTUAL TABLE IF NOT EXISTS filings_fts USING fts5(
     subject, category, type,
     content='filings', content_rowid='id'
 );
+
+CREATE TABLE IF NOT EXISTS api_keys (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    key TEXT UNIQUE NOT NULL,
+    name TEXT NOT NULL,
+    email TEXT NOT NULL,
+    plan TEXT NOT NULL DEFAULT 'free',
+    rate_limit INTEGER NOT NULL DEFAULT 30,
+    requests_today INTEGER DEFAULT 0,
+    requests_month INTEGER DEFAULT 0,
+    last_request_at DATETIME,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    expires_at DATETIME,
+    active INTEGER DEFAULT 1
+);
+
+CREATE TABLE IF NOT EXISTS usage_log (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    api_key_id INTEGER REFERENCES api_keys(id),
+    endpoint TEXT NOT NULL,
+    method TEXT NOT NULL,
+    status_code INTEGER,
+    response_time_ms INTEGER,
+    ip TEXT,
+    created_at DATETIME DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE TABLE IF NOT EXISTS usage_daily (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    api_key_id INTEGER REFERENCES api_keys(id),
+    date TEXT NOT NULL,
+    requests INTEGER DEFAULT 0,
+    UNIQUE(api_key_id, date)
+);
 `