feat(api+web): stream topic chat live over server-sent events

GET /v1/topics/:name/stream opens an SSE firehose, polled server-side
every 2s and streamed as `message` events. Forward-only — clients
hit /messages once for backfill, then live from connect-time onward.
Heartbeats every 30s keep the connection through proxies.

Web chat panel reads the stream via fetch + ReadableStream so the
bearer token stays in the Authorization header (EventSource can't
set custom headers, which would force token-in-URL leaks). Auto-
reconnect with exponential backoff. setInterval polling removed.

Vercel maxDuration bumped to 300s on the catch-all API route so
streams aren't cut at the 10s default.

drizzle migrations/meta/ deleted — superseded by the filename-
tracked custom runner in apps/broker/src/migrate.ts (c2cd67a).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/db/migrations/meta/_journal.json

@@ -1,90 +0,0 @@
-{
-  "version": "7",
-  "dialect": "postgresql",
-  "entries": [
-    {
-      "idx": 0,
-      "version": "7",
-      "when": 1775336269295,
-      "tag": "0000_living_namora",
-      "breakpoints": true
-    },
-    {
-      "idx": 1,
-      "version": "7",
-      "when": 1775339743477,
-      "tag": "0001_demonic_karnak",
-      "breakpoints": true
-    },
-    {
-      "idx": 2,
-      "version": "7",
-      "when": 1775340519054,
-      "tag": "0002_vengeful_enchantress",
-      "breakpoints": true
-    },
-    {
-      "idx": 3,
-      "version": "7",
-      "when": 1775463897329,
-      "tag": "0003_add-presence-summary",
-      "breakpoints": true
-    },
-    {
-      "idx": 4,
-      "version": "7",
-      "when": 1775468683383,
-      "tag": "0004_add-presence-display-name",
-      "breakpoints": true
-    },
-    {
-      "idx": 5,
-      "version": "7",
-      "when": 1775470435032,
-      "tag": "0005_add-presence-session-pubkey",
-      "breakpoints": true
-    },
-    {
-      "idx": 6,
-      "version": "7",
-      "when": 1775470979207,
-      "tag": "0006_add-sender-session-pubkey",
-      "breakpoints": true
-    },
-    {
-      "idx": 7,
-      "version": "7",
-      "when": 1775476994511,
-      "tag": "0007_add-presence-groups",
-      "breakpoints": true
-    },
-    {
-      "idx": 8,
-      "version": "7",
-      "when": 1775477883426,
-      "tag": "0008_add-state-and-memory",
-      "breakpoints": true
-    },
-    {
-      "idx": 9,
-      "version": "7",
-      "when": 1775480008546,
-      "tag": "0009_add-file-tables",
-      "breakpoints": true
-    },
-    {
-      "idx": 10,
-      "version": "7",
-      "when": 1775480729014,
-      "tag": "0010_add-context-and-tasks",
-      "breakpoints": true
-    },
-    {
-      "idx": 11,
-      "version": "7",
-      "when": 1775481222701,
-      "tag": "0011_add-streams",
-      "breakpoints": true
-    }
-  ]
-}