fix(schema): re-sync schema.sql with the chats-overview migration; make the drift gate required

From the PR description

TL;DR

The Schema drift gate ("Fresh install vs upgraded deployment") has been red on main since 57d754c. This PR makes backend/schema.sql carry the exact text of the get_chats_overview definition that migration 20260812_01_collection_pagination_queries.sql ships - a formatting-only change, no semantic difference - which restores the fresh-install and upgrade paths to identical fingerprints. It also proposes making this check required on main (instructions below - needs repo admin).

Root cause

57d754c ("fix: harden collection pagination and directory loading") rewrote get_chats_overview in the migration with the ownership predicate split across four lines:

     or (
       p.id is not null
       and p.user_id = p_user_id
     )

while schema.sql kept the same predicate on one line. Postgres stores SQL-language function bodies as opaque strings, so the drift gate - correctly - compares them textually: same semantics, different bytes, fingerprints diverge. Every push to main and every PR rebased onto it has carried the red check since (e.g. #251's rebase inherited it).

The fix

Exactly what the gate's own failure message prescribes: a migration was merged without being folded into schema.sql, so update schema.sql - never the already-shipped migration. After this change the entire get_chats_overview definition is byte-identical between backend/schema.sql and the migration (verified by extracting and diffing both definitions). One file, formatting only; the deployed function this produces is unchanged.

Verification

The Schema drift workflow runs on this PR - its result on this branch is the test, since the failing comparison (baseline + all migrations vs. this schema.sql) is exactly what the job executes.

Making this check required (needs admin - @willchen96)

This incident is the argument: the gate caught the drift immediately, but nothing stopped the merge, so main stayed red and every open PR inherited a failure it didn't cause. The Protect main ruleset currently requires a PR with one approval but no status checks at all. To make this one blocking:

1. Merge this PR first (flipping the switch while the check still fails would leave the fix itself hard to land, and other PRs stay red until they pick up the fix from main).
2. Settings → Rules → Rulesets → "Protect main" → Add a new rule: "Require status checks to pass" → add check Fresh install vs upgraded deployment (source: GitHub Actions). Optionally enable "Require branches to be up to date before merging" so the comparison always runs against current migrations.
3. While there, consider also requiring the other gates that already run on every PR (Backend build and tests, Frontend build and tests, dependency-audit (root/backend/frontend), gitleaks (full history)) - today none of them block a merge either.

I'd have flipped it directly, but ruleset edits need admin on the repo and this account only has push.

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