Record which embedder wrote the stored vectors and warn on a swap (#378)

The embedder moved from paraphrase-multilingual-MiniLM-L12-v2 to
multilingual-e5-small. Both are 384-dimensional, so nothing in the code
noticed: cosine between an old stored vector and a new query vector is
noise, and recall degrades silently.

So the DB now records the embedder that wrote its vectors. One value for
the whole DB (migration #11, a small `meta` key/value table) rather than a
column on every vector row: the backfill re-embeds every note and fact in
one pass, so a per-row marker would hold the same string in every row and
cost a column on two tables for nothing.

The identity comes from the embedder itself via a new optional ID() method
("multilingual-e5-small@384", model file name plus dimension), so pointing
the config at another model changes the string without anyone editing a
constant. mavend logs a loud WARNING at startup naming both the stored and
the configured embedder when they differ.

Detection only — recall behaviour is unchanged. TODO(#378) in
store.CheckEmbedder marks where the backfill will hook in.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CGeSZxh1DCtRxmFVSYVGvJ

internal/store/migrations.go

@@ -83,6 +83,11 @@ ALTER TABLE reminders ADD COLUMN next_fire_ts INTEGER;`, // #2
 		expires_ts INTEGER NOT NULL
 	);
 	CREATE INDEX IF NOT EXISTS idx_dialogue_sessions_expires ON dialogue_sessions (expires_ts);`, // #10 — the follow-up session survives a restart (Vikunja #363); small, TTL-pruned table, not a history log
+
+	`CREATE TABLE IF NOT EXISTS meta (
+		key   TEXT PRIMARY KEY,
+		value TEXT NOT NULL
+	);`, // #11 — small key/value table for facts about the DB itself; first key is embedder_id (Vikunja #378)
 }
 
 // migrate applies every migration with a number greater than the DB's current