[NSOC'26][GSSOC'26]Feat: Add Postgres Full-Text Search for issues

[diksha78dev]

Resolves #86.

Changes:

• Adds a Supabase migration to create a generated fts column (tsvector) and a GIN index on issues.
• Introduces search_issues RPC to enable searching with ts_rank sorting (since PostgREST does not support ordering by functions natively).
• Updates getIssuesPage to query the RPC when a search string is present, applying relevance ranking naturally without breaking existing filters.

Testing:

• Verified no lint, type, or test errors.
• Fallback gracefully drops special characters automatically via plainto_tsquery.

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