feat(backend): bootstrap Go module, config, and data plane

Introduce configuration validation, Postgres migrations, Redis/storage adapters, and the seed sample video.

backend/internal/db/migrations/0019_chat_export_tokens.sql

@@ -0,0 +1,17 @@
+-- Time-limited, revocable download tokens for project-wide chat export links.
+
+CREATE TABLE IF NOT EXISTS chat_export_tokens (
+    id           BIGSERIAL PRIMARY KEY,
+    token_hash   TEXT NOT NULL UNIQUE,
+    token_enc    TEXT NOT NULL,
+    label        TEXT NOT NULL DEFAULT '',
+    expires_at   TIMESTAMPTZ NOT NULL,
+    revoked_at   TIMESTAMPTZ,
+    created_by   BIGINT REFERENCES users(id) ON DELETE SET NULL,
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    last_used_at TIMESTAMPTZ
+);
+
+CREATE INDEX IF NOT EXISTS chat_export_tokens_active_idx
+    ON chat_export_tokens (expires_at DESC)
+    WHERE revoked_at IS NULL;