Remove unused server_timestamp() from IDatabase

[mkonopelski-gd]

What

Removes the dead IDatabase.server_timestamp() API surface across the DB layer.

Why

server_timestamp() returned a sentinel that write paths could drop into a field to mean "server stamps now-UTC at write." It is dead: nothing in the app calls it. The only references were docstring Example: blocks (interface.py, dependencies.py) and the database test suite — no executing caller. Code that stores timestamps passes real datetime objects, which every backend already normalizes. So the sentinel path is unused (EAFP/YAGNI).

Verified before removing:

grep -rn "server_timestamp" backend --include="*.py" | grep -v "app/database/" | grep -v "/test"
# (empty)

Changes

• app/database/interface.py — delete the server_timestamp abstractmethod; scrub stale Example: lines referencing it.
• app/database/firestore.py — delete the SERVER_TIMESTAMP impl (kept the firestore import; still used elsewhere).
• app/database/memory.py — delete the method, the _ServerTimestamp sentinel class, and the dead isinstance(value, _ServerTimestamp) branches in both _process_timestamps helpers (real datetime passthrough intact); drop the now-unused datetime/UTC import.
• app/database/dependencies.py — fix the get_db docstring example to a plain datetime.now(UTC).
• test/database/test_memory_db.py, test/database/test_firestore_db.py — delete the TestServerTimestamp classes and now-unused datetime imports.

Verification

• uv run pytest test/database/ -q → 45 passed, 15 skipped (firestore emulator skipped).
• Imports clean: uv run python -c "import app.database.interface, app.database.memory, app.database.firestore".
• Final grep for server_timestamp / _ServerTimestamp under backend/ → no matches.

Merge sequencing

The SQLite feature branch (feature/use-sqllite-instead-firestore, PR #13) carries a mirror _ServerTimestamp that will be dropped when that branch rebases onto this change. Please merge this PR first, then rebase PR #13.