feat: case-insensitive, whitespace-normalized name handles

[RagingRedRiot]

Why

For a name to work as a lookup handle it has to be unique and consistently normalized — otherwise one human-entered name can resolve to zero rows or to several.

This PR establishes that contract for usernames now and lays the schema groundwork so rooms benefits when room.rs lands. It contains no room-actor code itself — just the normalization rules and the room_name schema those queries will depend on.

What changed

• Case-insensitive uniqueness via LOWER() functional indexes on username and room_name. "Alice" and "alice" can't coexist, and a lookup resolves regardless of the casing the client sends. Stored casing is preserved for display.
• trim_ws() — a SQL helper that strips all leading/trailing whitespace (space, tab, newline, CR, …), unlike bare TRIM, which only removes spaces. Routed through every write and lookup so padded input can't create near-duplicate or unreachable rows.
• Empty → NULL for optional fields (first_name/last_name/alias): blank input collapses to NULL. On edit, an absent field is left can't be blanked.
• CHECK (col <> '') on every text column (users, rooms, messages) as a hard backstop, so '' can never be stored via any code path — present or future.

Writes and lookups go through LOWER() / trim_ws() consistently so they still hit the functional indexes.

Testing

• New tests/normalization.rs (16 tests): case-insensitive uniqueness + auth + lookup, casing preserved on read, whitespace-complete trimming (incl. tab/newline-only → rejected), empty→NULL on create, and the three-way edit semantics (none / blank / value).
• Full suite green: 85 passed.

Notes for reviewers

• tests/edit_user.rs has one intentional existing-test change: the empty_string_fields_overwrite_existing_values canary now expects None instead of Some(""), matching the new empty→NULL contract (its own comment predicted this). The CI test-modification check will flag it — that flag is expected here.
• room_name's index and CHECK are added ahead of room.rs so the room actor can rely on them; carry the same trim_ws($1) / LOWER(room_name) = LOWER(trim_ws($1)) convention into those queries.

[github-actions]

⚠️ Existing tests were modified

The following test files have removed or changed lines compared to main:

• tests/edit_user.rs (7 line(s) removed or changed)

Adding new tests is fine and does not trigger this notice. This appears when existing test code (or helpers under tests/common/) is edited or removed. That can be legitimate — but every change should be deliberate and visible to reviewers.