Add namespace-scoped performance indexes on projects

[nsheff]

Builds on query_improvements. Adds the indexes for the two namespace-scoped hot paths, complementing this branch's search / ORDER-BY indexes and N+1 fix. Added as model-level Index() to match the convention here.

Indexes added (Projects.__table_args__)

Index	Purpose
ix_projects_namespace_update_date on (namespace, last_update_date)	Namespace project listing (WHERE namespace = ? ORDER BY last_update_date DESC LIMIT n) → bounded index range scan instead of walking the global last_update_date index and filtering by namespace (the plan flip that scans most of the table for a namespace whose rows are not near the date tip), and no full sort.
ix_projects_ns_agg on (namespace) INCLUDE (number_of_samples, name) WHERE private IS FALSE	Namespace aggregation (SELECT namespace, count(name), sum(number_of_samples) ... GROUP BY namespace) → index-only scan (~25 MB) instead of a ~250 MB heap seq scan. Needs a fresh visibility map, so run VACUUM ANALYZE projects after.
trgm_index_namespace GIN on namespace	Namespace ILIKE search.

Why

The two namespace paths were full-scanning the ~250 MB projects heap on every request. On a RAM-starved host that means seconds-to-minutes of cold reads. These make both paths read a bounded, cacheable number of pages.

Applying to production (existing DB)

create_all (checkfirst) covers fresh installs. For the existing prod table, create concurrently so no blocking lock, then vacuum for the index-only scan:

CREATE INDEX CONCURRENTLY IF NOT EXISTS ix_projects_namespace_update_date
    ON projects (namespace, last_update_date);
CREATE INDEX CONCURRENTLY IF NOT EXISTS ix_projects_ns_agg
    ON projects (namespace) INCLUDE (number_of_samples, name) WHERE private IS FALSE;
CREATE INDEX CONCURRENTLY IF NOT EXISTS trgm_index_namespace
    ON projects USING gin (namespace gin_trgm_ops);
VACUUM ANALYZE projects;

Recommended cleanup while applying (not expressible via create_all), to cut write amplification:

DROP INDEX CONCURRENTLY IF EXISTS registry_index;              -- exact dup of projects_namespace_name_tag_key
DROP INDEX CONCURRENTLY IF EXISTS project_id_samp_index_ccnew; -- leftover invalid index from an interrupted REINDEX

Notes

• The new Index() DDL was verified to compile cleanly against the PostgreSQL dialect.
• Index changes alone are mitigation; they do not cure the underlying host RAM/EBS-bandwidth constraint (tracked separately).