feat: encrypt gitsshkeys.private_key at rest via dbcrypt (#25872)

Adds an optional dbcrypt wrapper around gitsshkeys.private_key. The
column is encrypted on insert and update through enterprise/dbcrypt when
external token encryption is configured, and decrypted on read.

A new private_key_key_id column references
dbcrypt_keys(active_key_digest) so revocation safety is enforced by the
existing foreign key. Rows with a NULL key_id stay plaintext and remain
readable. Existing plaintext rows can be backfilled by running `coder
server dbcrypt rotate`.

Generated with assistance from Coder Agents.

coderd/database/migrations/000515_gitsshkeys_private_key_key_id.down.sql

@@ -0,0 +1,3 @@
+ALTER TABLE gitsshkeys
+    DROP CONSTRAINT gitsshkeys_private_key_key_id_fkey,
+    DROP COLUMN private_key_key_id;

coderd/database/migrations/000515_gitsshkeys_private_key_key_id.up.sql

@@ -0,0 +1,7 @@
+ALTER TABLE gitsshkeys
+    ADD COLUMN private_key_key_id TEXT;
+
+ALTER TABLE ONLY gitsshkeys
+    ADD CONSTRAINT gitsshkeys_private_key_key_id_fkey FOREIGN KEY (private_key_key_id) REFERENCES dbcrypt_keys(active_key_digest);
+
+COMMENT ON COLUMN gitsshkeys.private_key_key_id IS 'The ID of the key used to encrypt the private key. If this is NULL, the private key is not encrypted.';