feat(coderd/database): add boundary_sessions and boundary_logs tables (#25441)

RFC: [Bridge ↔ Boundaries Correlation
RFC](https://www.notion.so/coderhq/Gateway-and-Firewall-Correlation-RFC-31ad579be592803aa8b3d48348ccdde9)

Add up/down migrations and matching sqlc queries for persisting Boundary
audit events, as specified in the Bridge/Boundaries Correlation RFC.

**Tables:**
- `boundary_sessions`: session metadata with `workspace_agent_id` FK,
`confined_process_name`, and timestamps (`started_at`, `updated_at`). ID
is externally supplied by the Boundary process (no DB-side default).
Created lazily when the first log for a session arrives.
- `boundary_logs`: individual audit events with `session_id` FK,
`sequence_number` (INT, primary ordering key), protocol/method/detail
fields, and `matched_rule` (nullable; non-NULL implies allowed).

**Indexes (per RFC):**
- `(session_id, sequence_number)` for the ordering query path
- `(captured_at)` for the retention purge path

**Queries:**
- `InsertBoundarySession` / `GetBoundarySessionByID`
- `InsertBoundaryLog` / `GetBoundaryLogByID`
- `ListBoundaryLogsBySessionID` with nullable `seq_after`/`seq_before`
exclusive bounds for fetching events between two known interception
sequence numbers
- `DeleteOldBoundaryLogs` with row limit to avoid long-running
transactions

**Also includes:** dbgen helpers (`BoundarySession`, `BoundaryLog`),
dbauthz implementations (reads gated on `ResourceAuditLog`, deletes on
`ResourceSystem`), and all generated wrappers (dbmock, dbmetrics).

No callers yet. A follow-up PR will add the dedicated `boundary_log`
RBAC resource type.

> Generated by Coder Agents

coderd/database/migrations/testdata/fixtures/000507_boundary_sessions_and_logs.up.sql

@@ -0,0 +1,35 @@
+INSERT INTO boundary_sessions (
+    id,
+    workspace_agent_id,
+    confined_process_name,
+    started_at,
+    updated_at
+) VALUES (
+    'a1b2c3d4-e5f6-4890-abcd-ef1234567890',
+    '45e89705-e09d-4850-bcec-f9a937f5d78d',
+    'claude-code',
+    '2026-04-01 10:00:00+00',
+    '2026-04-01 10:00:00+00'
+);
+
+INSERT INTO boundary_logs (
+    id,
+    session_id,
+    sequence_number,
+    captured_at,
+    created_at,
+    proto,
+    method,
+    detail,
+    matched_rule
+) VALUES (
+    'b2c3d4e5-f6a7-4901-bcde-f12345678901',
+    'a1b2c3d4-e5f6-4890-abcd-ef1234567890',
+    0,
+    '2026-04-01 10:00:01+00',
+    '2026-04-01 10:00:00+00',
+    'http',
+    'GET',
+    'https://api.anthropic.com/v1/messages',
+    'domain=api.anthropic.com'
+);