feat: anchor snippet creation timestamps on Stellar blockchain

[AgilityB]

On-Chain Snippet Timestamp Verification
Anchors snippet creation timestamps to the Stellar blockchain, providing immutable proof-of-existence for ownership and originality verification.

What changed
stellar.ts
Replaced the mock stub with real Stellar SDK integration.

submitHashToStellar — builds a Stellar transaction with a manageData operation storing the content hash and a Memo.text encoding the snippet ID + hash. Accepts createdAt so the snippet's original creation timestamp is permanently embedded on-chain.
submitBatchHashToStellar — submits up to 64 snippets in a single transaction using one manageData op per snippet.
Both functions fall back to a deterministic mock when STELLAR_SECRET_KEY is not configured, so local dev works without a funded account.
db.ts
Added four functions to support the verification flow:

getSnippetWithHash — fetches a snippet including the new verification columns.
storeSnippetHash — persists on_chain_hash, transaction_hash, and verified_at. Rejects any attempt to overwrite an existing record, enforcing immutability at the DB layer.
verifySnippetIntegrity — recomputes the current content hash and compares it against the stored on-chain hash to detect tampering.
getVerifiedSnippets — returns all snippets that have been anchored on-chain.
app/api/snippets/[id]/verify/route.ts

POST now passes snippet.created_at into submitHashToStellar, anchoring the original creation timestamp (not just the content hash) on-chain. Returns 409 if the snippet is already verified.
GET response now includes createdAt alongside the existing verification fields.
add-verification-columns.sql
Migration script adding on_chain_hash, transaction_hash, and verified_at columns to the snippets table, with partial indexes for efficient querying of verified snippets.

How to deploy
Run the migration against your NeonDB instance:
psql $DATABASE_URL -f scripts/add-verification-columns.sql
Set STELLAR_SECRET_KEY in your environment (testnet or mainnet key depending on NEXT_PUBLIC_STELLAR_NETWORK).
Testing

Anchor a snippet on-chain

curl -X POST http://localhost:3000/api/snippets//verify

Check integrity

curl http://localhost:3000/api/snippets//verify

List all verified snippets

curl http://localhost:3000/api/snippets/verify

Closes #76

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